The invention particularly applies to the manufacture of neutron-absorbing rods intended to be used in light water reactors (LWRs), and notably in Pressurized Water Reactors (PWRs).
Neutron-absorbing rods are usually grouped into control clusters. Among these clusters some are frequently moved within and rub against guides when in operation. These clusters also vibrate under the effect of the flow of water. The rods contained therein therefore risk becoming worn and losing part of their neutron-absorbing capacity, the very essence of reactor safety. The claddings and end plugs of the neutron absorbing rods are particularly exposed to this risk of wear.
The frequency and amplitude of the movements of some of these absorber rods, in particular when the reactor is used in load-following mode, the frequency and amplitude of the vibrations of some of the absorber rods, in particular for clusters remaining in stationary position, are such that is necessary frequently to control and ensure early replacement of a certain number of clusters having regard to the wear arising from friction.
To combat this wear, it has been proposed to harden the outer surface of the claddings by nitriding. Documents FR-2 604 188, EP-446 083, EP-537 062 and EP-801 142 describe plasma nitriding steps.
Such nitriding steps provide efficient protection against the wear of the claddings of the absorber rods.